Research On High Spatial Resolution Distributed Fiber Sensing Technique Based On Brillouin Effect

Distributed fiber sensors based on Brillouin effect are utilized to measure temperature and strain for the characteristic that the Brillouin scattering light is sensitive to the temperature and strain. High spatial resolution sensing techniques have great potential in the security monitoring of aerospace industry, major disaster and large infrastructure. Due to the limitation of phonon lifetime, the spatial resolution of Brillouin fiber sensor is limited to be over lm. To overcome the limitation, a novel BOTDR technique based on differential spectrum technique is proposed in this paper, which can achieve decimeter-scale spatial resolution while the sensing accuracy and sensing range are kept at acceptable level. The study and innovation of this paper are listed below:(1) The research background and developments of the high spatial resolution distributed fiber sensing techqiue are introduced. The significance of BOTDR technique and high spatial resolution BOTDR sensor in fiber sensing area are emphasized. The relations between Brillouin scattering light and temperature/strain are introduced. Meanwhile, the structure and the common technique of detecting Brillouin signal and extracting Brillouin frequency shift in the BOTDR sensing system are illustrated. That provides important theoretical foundation for design of high spatial resolution BOTDR system.(2) Through the therectical analysis of the FFT spectrum analysis technique, the energy distribution of spontaneous Brillouin scattering light within the corresponding region of the pulse is obtained, which is described by the weighting factor. The relation between the pulse width, FFT time window length and weighting factor distribution is obtained through theorectical analysis, based on which we proposed a novel two-step differential Brillouin spectrum technique and its feasibility analysis is discussed.(3) The high spatial resolution sensing system is setup and tested in the temperature and strain experiments. The feasibility of two-step differential Brillouin spectrum technique and the reduction effect of the pulse width difference to the SNR of the measured values are analyzed.40cm spatial resolution and 4.1MHz frequency resolution are achieved within the sensing ranged of 7.8km in the experiments. And the sensing length is enlarged to 25.4km with the use of repeater optical amplifiers while the spatial resolution is kept at 0.4m and frequency resolution is kept at 4.5MHz.